1. Field of the Invention
This invention relates to a method for the production of acrylic acid which, at a step of absorbing acrylic acid, enhances the absorption ratio of acrylic acid by introducing a low boiling substance-containing solution through a portion different from the top of an absorption column.
2. Description of the Related Art
Commercial production of acrylic acid generally resorts to the method of propylene oxidation which consists in subjecting propylene and/or acrolein to catalytic gas phase oxidation. When acrylic acid is produced by this method of propylene oxidation, the step of propylene oxidation gives rise to such impurities as water, acids like propionic acid, acetic acid, and maleic acid, and aldehydes like acetone, acrolein, furfural, and formaldehyde in the form of by-products. The gas containing these by-products is absorbed as an acrylic acid-containing solution generally via contact with an absorbent. This solution is subsequently purified by separating the absorbent by such a means as distillation and further separating low boiling substances and high boiling substances. The minute amount of such impurities as aldehydes which cannot be easily separated by distillation is possibly purified by a chemical treatment or a process of crystallization. However, the purification to a high degree necessitates many steps and complicates equipment and operation and forms one cause for degrading the yield of acrylic acid.
One known method, for example, produces acrylic acid of high purity by absorbing an acrylic acid-containing gas resulting from catalytic gas phase oxidation with a high boiling solvent, distilling the acrylic acid-containing solution thereby separating it into the solvent and crude acrylic acid, and subsequently subjecting the crude acrylic acid to a process of crystallization (JP-A-1997-227445). This method, however, forms a complicated procedure which comprises a step of cooling an acrylic acid-containing gas with a venturi, and then a step of absorbing the cooled gas and subsequently a step of removing low boiling substance, and thereafter a step of separating a high boiling solvent and crude acrylic acid by a distillation column.
If the step of acrylic acid production is enabled to treat the acrylic acid-containing solution of high concentration, it will enjoy the efficiency of decreasing the amount of treatment at the subsequent step of purification. Thus, one known method comprises supplying a reaction composition containing more than 7 vol % of propylene, molecular oxygen, and steam and the balance of an inert gas to a reaction vessel having disposed therein numerous reaction tubes each formed of two reaction zones packed with a catalyst thereby utilizing a propylene reactant of high concentration (JP-A-2000-103761). In a working example 2 cited in the official gazette, the absorption with water obtained an acrylic acid-containing solution having an average concentration of 73.8 wt %.
Another method has been disclosed which comprises introducing an acrylic acid-containing gas into an absorption column, introducing a recovery water containing acetic acid emanating from the bottom liquid of a solvent recovering column at the purifying step into the top of the absorption column thereby effecting absorption of acrylic acid therein, and producing as the bottom liquid of the absorption column an acrylic acid-containing solution composed of 50-80 wt % of acrylic acid, 2-5 wt % of acetic acid, and the balance of water (JP-A-1993-246941). This method obtains purified acrylic acid by subjecting the acrylic acid-containing solution to azeotropic dehydration using a mixed solution of two or more azeotropic solvents and subsequently passing the product of dehydration through such steps as the removal of high boiling substance.
Still another method has been disclosed which, in the absorption with water of an acrylic acid-containing gas resulting from a reaction of catalytic gas phase oxidation, comprises supplying the recovery water emanating from the step of azeotropic dehydration to the absorption column, supplying the resultant acrylic acid-containing solution to a stripping column, and obtaining an acrylic acid solution composed of 70.9 wt. % of acrylic acid, 25.6 wt. % of water, and 2.0 wt. % of acetic acid via the bottom of the stripping column (JP-A-2001-199931). This method obtains purified acrylic acid by performing azeotropic dehydration of the acrylic acid-containing solution and subsequently subjecting the product of dehydration to the step of crystallization.
The method disclosed in the official gazette mentioned above, however, is such that when an organic solvent is used as the absorbing solvent, this method necessitates a subsequent step of separating the solvent. In spite of this addition to the process, the method does not deserve to be regarded as producing the acrylic acid-containing solution of sufficiently high concentration. An attempt to obtain in the bottom liquid of the absorption column an aqueous acrylic acid solution containing acrylic acid in high concentration results in lowering the absorption efficiency and proves commercially infeasible. Such is the true state of affairs.
Thus, it is an object of this invention to provide a method for producing acrylic acid in high concentration by absorbing acrylic acid at high yield.
And, it is an object of this invention to provide a method for producing acrylic acid of high purity at a high yield by a simple process from the acrylic acid-containing solution mentioned above as the raw material.